WASHINGTON, Nov 29 (Reuters) - U.S. Agriculture Secretary Tom Vilsack on Wednesday said China took advantage of lower prices for corn in Brazil, leading to a nearly 20% drop in U.S. exports to China, but he expected the numbers to rise again over time.

At the same time, he told Reuters in an interview, the U.S. government was working hard to reduce American exporters' overreliance on China and other big markets and encourage greater diversification.

"The reality is that our market has been a little tight ... and our prices are a little higher than our friends in Brazil and in South America, and so as a result, China is taking the opportunity as they often do, to take advantage of low cost," Vilsack said. "Over time, we'll continue to see that righting and balancing of that."

He gave no specific details.

Exports of U.S. agricultural and related products to China through September totaled about $19.9 billion, the slowest pace in three years and down 18% from the January-to-September period last year, according to U.S. Census Bureau trade data. Exports to all countries are only down 12%.

Vilsack, the former governor of farm state Iowa, underscored the importance of diversifying U.S. agricultural exports.

"It's important for the United States to take this opportunity to understand the importance of diversifying its market opportunities away from an overreliance on one or two markets," he said.

Oilseeds and grains are the top U.S. export to China, accounting for $25.4 billion last year, far ahead of other goods such as semiconductors. But Brazil has been eating into the U.S. share of the Chinese market after harvesting bumper crops of soybeans and corn. (Reporting by Andrea Shalal, Karl Plume and Leah Douglas; editing by Jonathan Oatis)